In recent years optical fiber has become the medium of choice for many long haul signal transmission applications, and transmission rates have steadily increased, with a commercial system capable of operation at about 1.7 Gbit/sec available now. The bandwidth of commercially available single mode optical fiber however is enormous, and even in the currently available or projected highest bit rate systems much fiber bandwidth remains unutilized. For obvious economic reasons, it would be desirable to utilize more of the available bandwidth of optical fiber.
One way of increasing the bit rate of optical fiber systems is to multiplex two or more signal channels onto a single fiber, and systems using wavelength division multiplexing are known. It is also known that some forms of time-division multiplexing can be used in optical communication systems. See, for instance, U.S. Pat. No. 3,506,834, which discloses a time-division multiplexed system that could in principle use optical fiber as the transmission medium. As is well known, in a time-division multiplexed communication system each of n (n.gtoreq.2) data streams is allocated a series of time slots on the multiplexed channel, with a multiplexer (MUX) assembling the higher bit-rate stream from the data streams, and a demultiplexer (DEMUX) reconstructing the data streams by separating bits in the multiplexed stream.
Time-division multiplexed optical fiber systems can typically be divided into electrical and optical time-division multiplexed systems. In the former, multiplexing is carried out in the electrical domain, before the electrical/optical (E/O) conversion of the multiplexed signal, and demultiplexing is also carried out in the electrical domain, after the optical/electrical (O/E) conversion of the multiplexed signal. Such a system requires electronics that is capable of processing a very high bit rate stream of electrical pulses, and limitations of the electronics typically limit the possible bit rate of such systems. Electronic "bottlenecks" potentially can occur in the transmitter between and including the MUX and the E/O converter, and in the receiver between and including the O/E converter and the DEMUX, where the electronics must operate at the full multiplexed bit rate.
In optical time-division multiplexed (OTDM) lightwave communications systems these electronic "bottlenecks" are removed by moving the E/O and O/E converters into the baseband channels. Multiplexing is carried out after the E/O conversion and demultiplexing before the O/E conversion. See, for instance, T. S. Kinsel et al., Proceedings of the IEEE, Vol. 565(2), Feb. 1968, pp. 146-154. Of course, hybrid systems are also possible. For instance, L. C. Blank et al., Electronics Letters, Vol. 23(19), Sept. 1987, pp. 977-978, discloses a system in which multiplexing is carried out after E/O conversion but in which the multiplexed signal is demultiplexed after O/E conversion.
Even though the principle of OTDM has been know for some time, implementation of such systems have been relatively slow, especially for high bit rate systems. One reason for this slow progress may have been the difficulty of providing an appropriate timing signal at the receiver. Another reason may have been the difficulty of providing an optical switch that can separate a very high bit rate pulse stream (typically the multiplexed stream) into two or more pulse streams without causing unacceptably high levels of cross talk. Optical switches are discussed, for instance, in K. Habara et al., Electronics Letters, Vol. 21, 1987, pp. 631-632, and in S. K. Korotky et al., IEEE Journal of Lightwave Technology, Vol. LT-3(1), 1985, pp. 1-6.
Significant progress has recently been made, resulting inter alia in the demonstration of a 16 Gbit/s OTDM system having four 4 Gbit/s channels. See R. S. Tucker et al., Electronics Letters, Vol. 23(24), pp. 1270-71; and R. S. Tucker et al., Journal of Lightwave Technology, Vol. 6(11), pp. 1737-1749; both incorporated herein by reference. See also G. Eisenstein et al., Electronics Letters, Vol. 23(21), pp. 1115-1116; and R. S. Tucker et al., Electronics Letters, Vol. 23(5), pp. 208-209. These papers disclose OTDM systems in which the timing signal for demultiplexing is derived from a fully demultiplexed pulse stream. Specifically, these prior art OTDM systems comprise clock means for producing a substantially sinusoidal electrical signal which is used to drive the switch or switches that are part of the DEMUX portion of the receiver means, with the clock input derived from one of the receivers. The clock input signal thus has a strong component at the (4 GHz) baseband frequency, and it is this component that is utilized for timing recovery.
Although these prior art systems have been successfully operated there still remain some potential problems. For instance, the systems are likely to experience instability, especially during start-up, since the initial phase relationship between the clock input signal and the multiplexed optical pulse stream is generally unknown. Consequently start-up of such a system frequently requires relatively complex adjustments. In addition, noise and crosstalk between baseband channels may cause timing errors which can result in a loss of synchronization.
In view of the obvious economic significance of optical communications systems that can be operated at very high bit rates, it would be highly desirable to have available an OTDM system which can be less subject to instability during start-up and more immune to crosstalk and noise than prior art systems. This application discloses such a system.